Dot matrix print heads of the impact type are well known and are widely used in computer printers. They are generally comprised of a plurality of print wires each driven by a solenoid. The wires are typically of the ballistic type, being propelled in free flight by an impulse force directly proportional to the coil current and the magnetic field strength. The impulse force must propel the wire to the required velocity and kinetic energy to preferably print ink dots on multiple sheets of paper. Print wires are typically slender and must be supported to prevent buckling. The wires are arranged with adequate supporting guides to merge towards the front to form a closely packed array of printing tips aligned by a nose bearing in a matrix of inline or staggered rows and columns.
After printing the selected dots in each column, or vertical straight line, the printhead mounted on a carriage is stepped either discretely or continuously to the next horizontal printing position. Correspondence quality print and graphics are produced by multiple traversing of the printhead while selectively impulsing combinations of print wires. It is typical to print using a head comprising a single or double column of seven to nine wires. However, three staggered columns of eight fine wires are typically required for producing letter quality print and high resolution graphics in a single traverse. Using smaller diameter print wires enables more effectively overlapping the dots without having the edges of the characters give a ragged appearance so as to affect the print quality.
Because of the high resolution printing capability with computer software control, the dot matrix printing concept can provide the desired flexibility not possible with fully-formed character printing. However, dot matrix printheads typically must print more slowly as the dot density or the graphic resolution is increased to enhance print quality.
Conventional dot matrix printheads tend to be speed limited because of the large mass or inertia of the print wire mechanism coupled with the difficulties of dissipating heat and the long response time of solenoid devices.
Presently, moving coil printhead actuator designs are also speed limited and may rely on pulse damping to prevent backstop rebound. The actuators are generally thin and planar but with some mass and stiffness, and may have a copper coil formed on an insulating substrate. These low cost printheads generally comprise seven to nine actuators closely stacked in parallel planes in one primary magnetic flux gap with the actuator printing tips disposed along a common line.
Heat transfer from the coils to the magnet heat sink area tends to be markedly decreased when more than about three actuators are closely stacked in parallel planes. Difficulties in adequately dissipating the heat generated tends to impose limitations on this design configuration. Substrate coil forms are not densely packed and interact less effectively with the magnetic flux in the air gap. Magnetic field strength is lower in one more widely spaced air gap.
Whenever magnetic field strength is decreased, or coil interaction is less effective, or actuator mass is increased, a larger coil current is required to obtain the necessary kinetic energy and velocity for printing. The heat that must be dissipated is directly proportional to the square of the current. With a lesser heat transfer capability the allowable maximum operating temperature is reached at a lower current level. This further limits speed and performance.
Moving coil dot matrix actuators have the potential for achieving high speeds provided the actuator mass and support stiffness is small, the coil inductance is low, the heat dissipation is adequate, and the magnetic circuit is cost effectively optimized to achieve the highest possible flux density in the air gap. Optimizing the quantity of high energy permanent magnetic material in the circuit requires the design to operate at the flux density where the available external energy is at a maximum, or the peak energy product of the magnetizing force of the magnet and the flux density of the magnet.
A large number of computer data and word processing applications exist where it is very desirable to have a fast versatile, cost effective printer that has the capability of producing both high resolution graphics and letter quality documents.